1. Field of the Invention
The present invention relates to a sintered body consisting mainly of beta alumina used as a solid electrolyte (hereinafter, abbreviated as beta alumina sintered body) and a method of manufacturing the same, and especially relates to a beta alumina sintered body used in a sodium sulfur secondary cell and a method of manufacturing the same.
2. Related Art Statement
Since a beta alumina sintered body shows a good sodium ion conductivity, it is used in the sodium sulfur secondary cell as a solid electrolyte for insulating metallic sodium as a cathode active material and molten sulfur as an anode active material. In this sodium sulfur secondary cell, a generation of almost all the inner resistance is due to the solid electrolyte. Therefore, in order to reduce an output power loss during a cell charge condition and also reduce a power loss during a cell discharge condition, it is preferred to minimize a relative resistance at a temperature such as 350.degree. C. on which the beta alumina sintered body is used. Moreover, since a life of the sodium sulfur secondary cell is dependent upon that of the solid electrolyte, it is preferred that a beta alumina sintered body has a high strength.
Generally, in order to make the relative resistance lower, a crystal phase of the sintered body is formed by a .beta." alumina which shows a better sodium ion conductivity as compared with .beta. alumina. To this end, oxides of metallic ions having less than two valences such as magnesium oxide, lithium oxide and the like are added therein so as to stabilize .beta." alumina phase.
However, a relative resistance obtained according to the method mentioned above is 3.0 .OMEGA..multidot.cm at 350.degree. C. at best, even if a composition is optimized. Moreover, a lower relative resistance can be obtained by promoting a grain growth and by decreasing a boundary resistance. However, in this case, a strength of the beta alumina sintered body is largely decreased, and thus the beta alumina sintered body can not be suitably used in a real use.
Further, as mentioned above, there is a drawback such that a temperature range for most suitably sintering the beta alumina formed body becomes narrow. That is to say, if the sintering temperature is beyond this temperature range, an extraordinarily grain growth is generated and thus a strength is decreased. Therefore, in this case, a relative resistance of the beta alumina sintered body can be decreased, but the beta alumina sintered body is not used in a real use. Actually, the beta alumina sintered body needs such characteristics that a bulk density is above 3.20 g/cm.sup.3 and a flexural strength by using four support points is above 180 MPa. Moreover, it is preferred that a temperature range for most suitably sintering the beta alumina formed body becomes wider and wider, because a temperature controlling during a sintering step becomes easier. This is particular for a mass production of the beta alumina sintered bodies using a large furnace, because a temperature distribution in the large furnace is largely varied.
In order to eliminate the drawbacks mentioned above, there is disclosed, in Japanese Patent Laid-Open Publication No. 59-141459, a technic such that an influence of calcium oxide are reduced or eliminated by adding zirconia into the beta alumina so as to improve an ion conductivity. Moreover, there is disclosed, in Japanese Patent Laid-Open Publication No. 60-251172, a technic such that a sintering temperature is lowered by adding zirconia. However, the beta alumina obtained according to the method mentioned above has also a relative resistance at 350.degree. C. of 3.0 .OMEGA..multidot.cm at best, even if a composition is optimized.
Further, in order to improve an endurability of the beta alumina sintered body, there is disclosed, in Japanese Patent Publication No. 52-3411 and in Japanese Patent Publication No. 52-40325, a technic such that tantalum or tantalum oxide is existent in the intergranular phase by adding tantalum in the beta alumina sintered body.
However, the beta alumina sintered body obtained according to the method mentioned above can improve the endurability, but a tantalum component is precipitated in the intergranular phase as a different phase. Therefore, a resistance in the intergranular phase is increased and the relative resistance becomes high.